1. Field of the Invention
The invention relates to microscope objectives, and more particularly the type of strong dry objective wherein the curvature of the field is corrected and the image field is flattened by a front lens consisting of a thick meniscus lens.
2. Description of the Prior Art
As is known over a century now, the various defects of an optical system in projecting can, at least partly, be overcome separately by different suitable means. One of such means is a rigidly symmetrical arrangement of all parts of the optical system, such as is the use of the so-called Gauss-type of photo objectives. Another possibility of separately correcting single picture defects was made known by Petzval about the mid-nineteenth century. Certain defects of this type are accumulated in one part of the lenses of the entire system and corrected in the other lenses. In the so-called Petzval objective which consists of two groups of lenses spaced widely apart from each other, the defect in the aperture and the asymmetrical defects of the entire system are corrected through the front lens member. Asymmetrical defects are those which enter the system just by being asymmetrically arranged. According to Petzval, the defects of the image field, e.g. preferably the astigmatism, are corrected through the rear lens. By skilled adaptation of the rear lens it is possible to transfer the diaphragm of the entire system to the front lens. The distance of the diaphragm is thereby reduced to zero for the front lens, with the effect that the shell defect is defined as a solid, invariable value by the contribution of the front lens to the Seidel sums, which defect can be corrected by suitable adaptation of the rear lens.
Further details, for instance, can be gleaned from the monograph of Georg Franke, "PHOTOGRAPHIC OPTIC", published in the Akademischen Verlagsgesellschaft, Frankfurt/Main, 1964, appearing on page 138.
These general techniques as known for the correction of the photo-objective have now been improved and applied to micro-objectives in the present invention.
Microscope objectives of the general type of this invention are shown, for instance, in German Pat. Nos. 821,126, 924,539, and 970,606, and additional objective lenses are provided in these optical systems to correct aberrations the correction of which has been made more difficult by the addition of the thick meniscus front lens.